You could cosplay as pretty much anypony who has a similar hairstyle to yours if you get that spray-on dye that comes off in the shower. Then wear an outfit the same colour as the pony, draw a cutie mark on an index card and cut around the shape, and glue the cutie mark to your hip. For a unicorn, make a horn out of something or other and put it on a headband, and for a pegasus, just make wings out of cardboard or something and pin 'em to your back. I've put a lot of thought into this, since I'm thinking of maybe doing a Pinkie Pie cosplay sometime. =3 Feel free to steal any of my ideas.